257
AWUS01 KWNH 271812
FFGMPD
INZ000-ILZ000-272100-

Mesoscale Precipitation Discussion 0861
NWS Weather Prediction Center College Park MD
211 PM EDT Mon Jul 27 2026

Areas affected...Northeast Illinois, Northwest Indiana

Concerning...Heavy rainfall...Flash flooding possible

Valid 271809Z - 272100Z

Summary...There will be a window for potential training of
thunderstorm activity from the Chicago Metro Area toward the
border region between northeast Illinois and northwest Indiana
between 1 PM and 4 PM CDT. Rain rates could exceed 2 inches per
hour at times; if those occur in areas that have already received
heavy rainfall, flash flooding could rapidly develop, or existing
flash flooding could worsen.

Discussion...Isolated supercells along the western shore of Lake
Michigan have occasionally exhibited enough southward right-moving
tendency near the composite lake breeze and outflow boundary to
produce rapid bursts of heavy rainfall on the order of 1-2 inches
in about 45 minutes. Thus far, the threat of flash flooding has
been more isolated as the storms themselves were spaced out, but
has materialized where supercell motion has been slowest and
intersected with urban areas. However, there are signals in the
past 30 minutes for some elevated thunderstorm redevelopment in
the wake of a lead supercell and storm cluster now on the south
side of Chicago. The redevelopment is occurring over the urban
corridor from Lake County into Cook County, and could pass over
some areas that have already received heavy rainfall. The lead
thunderstorm cluster should continue to make progress south along
the IL-IN border region with continued intense instantaneous rain
rates, and there is potential for a training band of convection to
continue to feed in behind it over the next few hours. The
environment will be very similar -- a reservoir of extreme
instability upstream with high PWs -- which will support rain
rates over 2 inches per hour and a continued flash flood threat.

Lamers

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...IWX...LOT...

ATTN...RFC...MSR...TIR...NWC...

LAT...LON   42438788 42278741 41488694 40828701 40868801
            41598827 42258823